As other Monks have answered the main bit of your question i.e. the memory is 'freed' but not 'released' - one tip that i have found useful for when i really do need to release memory is to separate the work of into a child process, and when this process finishes the memory really will be cleared. Introduction to Parallel::ForkManager may help you with this.

Some other useful stuff if you want to find out more:

Hope this helps